1. Field of the Invention
The present invention pertains to a process for applying a metal layer to surfaces of light metals, especially to surfaces of aluminum, magnesium and their alloys, to applications of the process for coating cylinder faces of internal combustion engines and rotationally symmetrical parts with layers having very high wear resistance, especially of valves, nozzles and other parts of high-pressure injection systems for motor vehicle engines as well as to a nanocrystalline iron-phosphorus layer.
2. Brief Description of the Related Art
In coating light metals, especially aluminum, magnesium and their alloys, considerable efforts were made in the past to optimize the desired surface properties of these metals for the applications being considered. These metals are relatively soft and have, in general, only insufficient tribological and corrosion properties, so that their fields of use are very limited without additional precipitation hardening of the surface, unless special and correspondingly expensive alloys, e.g., supereutectic AlSi, are used. There has been great interest for some time especially in the automotive industry in using light metals to reduce weight in order to reduce the fuel consumption. For example, engine blocks are manufactured from this material. The cylinder faces, in particular, now require special measures to meet the specifications. It was therefore necessary to find processes for improving the surface with which the desired properties can be obtained.
W. Paatsch reported in this connection in Metalloberfläche, Vol. 51 (1997), pp. 678–682, that the surface of aluminum materials can be improved in a suitable manner with nickel-phosphorus layers and nickel layers applied by electroplating, in which hard materials are embedded as a dispersion in the layer, e.g., with silicon carbide as the dispersed material, so that the surface properties of the light metal meet the requirements imposed on cylinder faces in internal combustion engines. These layers partly confer good corrosion resistance and a high degree of protection against wear on the surfaces. As an alternative, good wear properties can also be obtained with hard chromium layers on the light metal surfaces, optionally aftertreated according to a plasma nitriding process. As an alternative, it is also possible to use thermal spraying processes, e.g., the powder or wire spraying process. To reach a sufficient adhesive strength on the cylinder faces of the layers deposited according to these processes, the energy of the particles sprayed on must be as high as possible. Detonation spraying and the HVOF (High Velocity Oxygen Fuel) technique are therefore used. For example, tungsten carbide particles can be applied to the surfaces in a metal matrix, e.g., in a cobalt or cobalt-chromium layer, so that a very strongly adhering and especially particularly corrosion-resistant layer is formed. Tungsten carbide layers that have very good tribological properties can also be prepared by plasma spraying.
However, depending on the process according to which they were prepared, the said layers have various drawbacks: On the one hand, the preparation of these layers is extremely complicated and therefore expensive, so that they are not suitable for mass application, as in the automotive industry (especially detonation spraying and the HVOF technique). The described nickel-phosphorus layers deposited by electroplating lack sufficiently good tribological properties. This also applies to the above-mentioned silicon carbide dispersion layers. The latter have not proved successful as coatings for cylinder faces, because the emergency running properties of the engine, i.e., the ability of the engine to withstand a transient separation of the oil film on the faces without damage, were not satisfactory in this case. This was due to the unsatisfactory corrosion resistance of the layers at excessively low oil temperatures over a long time and/or to unsatisfactory resistance to fuels with high sulfur content and to the insufficient wear properties that are associated herewith.
An alternative coating system is described in DE 196 53 210 A1. These are corrosion-resistant iron layers that contain 0.02 wt. % to 0.5 wt. % of nitrogen. It is stated that these layers can be deposited on aluminum or its alloys by electroplating. The coating of the inner sides of aluminum cylinders of internal combustion engines are given as examples of the application of such layers. A deposition bath containing iron(II) ions is used to deposit the layers, and the layer is deposited electrolytically using an anode consisting of iron or preferably using an insoluble anode, which consists of a titanium plate having a film consisting of an oxide of ruthenium, iridium, tantalum, tungsten, rhodium, cobalt or manganese.
Another coating method is described in U.S. Pat. No. 5,368,719. Iron layers are deposited in this case according to electroplating processes on pistons of internal combustion engines, the pistons consisting of aluminum or its alloys. A bath containing iron(II) sulfate is used for this purpose. Graphite, lead, platinum and titanium are used as materials for the anodes.
U.S. Pat. No. 4,746,412 discloses a bath for depositing iron-phosphorus alloy layers. Such baths contain iron(II) ions, hypophosphorous acid, a hypophosphite, phosphorous acid or an orthophosphite and optionally boric acid or aluminum chloride. The layers obtained have a phosphorus content of 0.1 wt. % to 9.9 wt. %. The layers are applied, e.g., on the inner walls of pistons of internal combustion engines. According to the data contained in this document, the layers possess good tribological properties.
In many cases, the suitability of the baths used for the deposition for use in industrial processes is not satisfactory in mass production despite the great advances made in this area. For example, functional layers of uniform thickness must be able to be deposited with little effort on the inner walls of the cylinders, and the reproducibility and the constancy of the desired layer thickness is of particular significance as well. The adhesive strength of the functional layers applied to the light metal surfaces also fails to meet the specifications in all cases. This applies especially to the layers applied according to the plasma spraying method. Furthermore, the problem that the constancy of the conditions required for a qualitatively satisfactory mass production can be achieved during the process only with considerable efforts is encountered with the use of electroplating processes for depositing iron layers as well. Despite comprehensive monitoring and control methods for conducting the process, it has not yet become possible to guarantee a fine setting for the lasting constancy of the process parameters, so that it is not possible to eliminate variations in the quality of the deposited layers without problems. The documents mentioned contain no references to these problems and to their solution.